1. Technical Field
The present invention relates to an arrangement for engine braking a motor vehicle that includes an internal combustion engine. Associated with the engine is at least one cylinder, at least one exhaust valve at the cylinder and a rocker arm for activation of the exhaust valve. The rocker arm is arranged on a hollow rocker arm shaft, and is further arranged to be effected by ridges on a camshaft. The device further includes a control valve for controlling oil pressure in the rocker arm shaft and means receptive to an increase of oil pressure in the rocker arm shaft that is integrated in the rocker arm for absorbing play between the rocker arm and the exhaust valve in the case of an increased oil pressure. In this case, at least one of the ridges then causes the opening of the exhaust valve with an engine braking effect. The invention also relates to a method for affecting engine braking using the above-described arrangement.
2. Background Art
Commercial vehicles such as trucks and buses are normally equipped with an engine braking function in order to spare the wheel brakes in the vehicle during braking (causing negative acceleration). In this context, it is known to provide an engine braking effect in an internal combustion engine by arranging a throttle device, for example in the form of a valve, in the exhaust system of the engine. In this way, a certain portion of the work expended during the exhaust stroke of the cylinder can be utilized to increase engine braking effects.
Another kind of engine brake is a so-called compression brake, which has the function of one or several of the engine""exhaust valves being controlled so that air which has been compressed during the compression stroke of the engine in the combustion chamber is allowed to partially flow or escape out into the exhaust system. This means that a part of the compression work carried out during the compression stroke is not utilized during the expansion stroke of the engine; feature that is also used to obtain a braking effect on the crank shaft.
In known compression brakes, the exhaust valves are controlled so that the camshaft of the engine is given a profile that is shaped with at least one extra ridge in order to obtain an opening of the exhaust valves with the intention of generating a braking effect. In addition, the valves are shaped with a small play, the size of which is chosen (together with the dimensions of the extra ridge) so that the extra ridge will not affect the valves during normal engine operation. For this purpose, the extra ridge has a lifting height that is very small compared to the ordinary exhaust ridge. In order to make the extra ridge operable during engine braking; that is so that the exhaust valves will be able to be open when engine braking, the respective rocker arm is adapted with a device in the form of a displaceable piston which is effected by oil to be positioned in an expanded position. This causes valve play to be eliminated, and the lifting height of the extra ridge then becomes sufficient in order to open the exhaust valves.
Apart from utilizing an engine braking device for braking the vehicle as such; that is, as a complement to the wheel brakes of the engine, there is also a desire to use a compression brake when shifting gears in the gearbox of the vehicle. This is apt to be an even more important aspect in commercial vehicles such as trucks and buses that are ever more frequently equipped with automatic or semi-automatic gearboxes. Such gearboxes can be likened to conventional manual gearboxes, with the difference being that the shifting of gears is carried out by means of a control device, instead of manually by the driver. FIG. 1 shows the principal phases in connection with an up-shift (i.e. to a higher gear) of such a gearbox. In that figure, a comparison between engine torque and engine speed is shown relative to time for a given type of engine.
As is evident from FIG. 1, phase xe2x80x9caxe2x80x9d shows a normal operating state before a gear shift is initiated. Phase xe2x80x9cbxe2x80x9d shows the removal of engine torque as soon as it has been decided that an up-shift is to take place. Phase xe2x80x9ccxe2x80x9d shows the release of a claw coupling (clutch) in order to de-couple the gearbox from the engine. Phase xe2x80x9cdxe2x80x9d shows a release of the engine speed in order to match the engine speed to the newly chosen gear ratio. As soon as the engine speed has been reduced sufficiently, the new gear can be engaged. Thus, phase xe2x80x9cexe2x80x9d shows the engagement of a new claw coupling. Phase xe2x80x9cfxe2x80x9d shows renewed application of torque, and phase xe2x80x9cgxe2x80x9d shows a normal operating condition after the gear shift has taken place.
In order to reduce loss of driving power of the engine during up-shift, it is an advantage if the engine speed can be matched to the new gear ratio as soon as possible. From document SE-0502154 C2 it is known to selectively introduce an exhaust brake during an up-shift when certain operating parameters are obtained, in order to achieve a rapid decrease of engine speed during the gear shifting process. In this way, it is alleged that wear on the exhaust brake system is decreased since the introduction of the exhaust brake only takes place during a small part of the total amount of the up-shift process.
An auxiliary brake system for commercial vehicles is known from U.S. Pat. No. 5,193,497 that shows an internal combustion engine equipped with a device for absorbing valve play in the valve mechanism of the engine. The absorption is carried out by means of an actively adjustable, hydraulically driven absorption means, adjustable between two positions, a contracted position, and an expanded position, which absorption means is arranged at the working end of a rocker arm whose valve play is to be absorbed. In a way that is described in detail hereinbelow, the existing pressurized oil system of the engine is used in order to guarantee the function of the device.
The auxiliary brake system that is described in the above-mentioned U.S. Pat. No. 5,193,497 has enjoyed considerable commercial success. Due to the relatively long time necessary for the absorption means to reach its extended position, however, the system according to U.S. Pat. No. 5,193,497 is not suitable for reducing engine speed at an up-shift.
From U.S. Pat. No. 6,544,143, which is expressly incorporated herein by reference, an arrangement is previously known for engine braking in an internal combustion engine. This arrangement is adapted for engine braking by the reduction of the engine speed when gear shifting, and for this purpose includes a special device that is receptive to a signal which is generated as a reply for a need to obtain a gear shift and for obtaining an absorption of a valve play in a rocker arm.
In connection with an engine brake device that is used when shifting gears, a pressure regulating valve can be utilized for supplying pressurized oil to a device for absorbing valve play in the rocker arm. Such a control valve device includes a displaceable valve body that can be affected to be positioned in a first position which corresponds to a pressure reducing function, and a second position which corresponds to a non-pressure reducing function. The oil is supplied to the rocker arm by means of a canal, which is provided with an exhaust in the shape of a very narrow hole through which oil can flow, and in this way be made to affect the valve body to, depending on operation, be positioned in any of the predetermined positions. For this purpose, the control valve is also provided with an adjustable magnet valve arranged for drainage of oil that has been fed through the narrow hole.
Although this previously known device in principle functions satisfactorily, it has some drawbacks. The main one is that it includes a small and carefully defined hole for the transport of oil, which causes a high sensitivity to clogging and tolerances. In addition, this previously known valve causes a relatively slow coupling and de-coupling, which is particularly noticeable in connection with gear shifting. In addition, the design is sensitive to external disturbances, for example in the form of temperature changes and pollution such as, for example, dirt particles or coatings.
A purpose of the present invention is to provide an improved device for engine braking, which in particular is suitable for utilization for a quick decrease of the speed of an engine during gear shifting.
This purpose is obtained according to the present invention through a device having a control valve that includes an adjustable valve body and a canal between the valve body and the rocker arm shaft which constitutes a controllable exhaust. The valve body is arranged to be positioned in a first position with a pressure reducing effect, and in a second position without a pressure reducing effect.
It is a further purpose of the invention to obtain a method for rapidly decreasing the speed of an engine during gear shifting.
As an example, this purpose is achieved by a method including control of a control valve having a controllable valve body and a canal between the valve body and the rocker arm shaft. A controllable exhaust is also provided with control accomplished by the positioning of the valve body in a first position with pressure reducing effect and a second position without pressure reducing effect.
By means of the invention, certain advantages are obtained. Most importantly, it can be mentioned that the invention permits a very rapid braking of an engine in connection with gear shifting. The invention also offers a very robust design with a reliable function.
Preferred embodiments of the invention will become evident by the appended dependent claims.